1. Field of the Invention
The invention relates to an improved piezoelectric sensor comprising one or more pairs of sensitive elements of great length.
Such a sensor may in particular be used as a hydrophone for different off-shore operations and, for example, for seismic sea prospection for receiving the seismic waves coming from the immersed sub-soil, which have been transmitted by a marine seismic source and reflected by the underground geological discontinuities.
2. Description of the Prior Art
The sensitive elements are formed of a substrate having piezoelectric properties and two electrodes disposed on each side of this substrate.
The substrate may also be in the form of ribbons or flexible strips made from a synthetic plastic material to which a treatment confers piezoelectric properties. Materials are generally used such as PVDF (polyvinylidene fluoride), polyethylene, PTFE (polytetrafluoroethylene) etc.
The flexible sensitive elements are very often used in the form of ribbons or strips disposed on a flat support or wound in a spiral about a cylindrical core. The ribbons or strips are generally combined in pairs so as to increase the sensitivity of the assembly. By disposing the ribbons or strips on each side of a support with opposite polarities and connecting them electrically, the parasitic voltages due to the flexing of the support and accelerations are compensated for.
By using flexible ribbons or strips, continuous hydrophones may be constructed of relatively great length which provide wave number filtering. Incorporated in seismic streamers towed by a ship, the flexible ribbons or strips filter out certain parasitic noises and facilitate reception of the signals reflected by the discontinuities of the immersed sub-soil, in response to seismic waves transmitted by a towed source.
In a known arrangement, a piezoelectric receiver is formed by winding a first sensitive strip associated with its two electrodes in a spiral about an insulating cylindrical core. An insulating flexible strip is superimposed by winding on the first sensitive strip. On top, a second sensitive strip is further wound in a spiral in the opposite direction to the first. Rings respectively contacting the electrodes of the two sensitive strips are connected by conducting wires so as to form a series interconnection.
In another known arrangement, between the two crossed spiral windings, a thin conducting ribbon may be interposed with interposed this conducting ribbon being transparent to the sound waves. The ribbon covers the side edges of all the turns of the lower winding and provides a good electric contact between its upper electrode and the lower electrode of the superimposed winding, which contributes to reducing the electric resistance of the connection and improves the electric reliability.
Piezoelectric sensors of the aforementioned type are described, for example, in French Patent No. 2,145,099 and corresponding U.S. Pat. No. 3,798,474 or Published French Patent Applications Nos. 2,601,132 and 2,603,422 respectively corresponding to U.S. Pat. Nos. 4,768,173 and 4,810,913.